What's the best way to cover my summerhouse roof. I've just finished putting the roof on, it's 5m x 3m and is made up of 28mm thick T&G planks so is fairly substantial. There is a fall of 1 in 50 down the 5m length. I have been advised with it being T&G and therefore prone to movement I would need an underlay felt down first and them two layers of mineral felt on top. Is this overkill, would one layer of good felt properly fitted really not work? I don't want to bodge it but 3 layers of felt is going to be difficult to work with IMO, especially on the corners.